Aspirations to harness the lethal potency of plant and bacterial toxins for beneficial use in medicine have yet to be fully realized. The proposed research addresses this critical problem from a novel perspective by deploying a model system of neoplastic disease which is curable using selective toxin therapy. Valuable information regarding toxin-based therapeutics will be obtained and this should facilitate the design of agents with optimal clinical effectiveness and utility. Various toxins have been covalently coupled to selective antibody and ligand carriers to focus their action exclusively on cancer cells. These new derivatized toxins provide very potent and highly specific cell kill in vitro but their performance as therapeutic agents in animal models falls far short of expectation. Substantial deficiencies have persisted despite years of intensive effort to refine the technology for producing toxin conjugates. This proposed study will identify the key factors underlying in vivo effectiveness. The therapeutic model will provide a frame of reference to judge the performance of modified toxins and to improve those which fail to achieve cures. Lastly, the system offers a means for directly testing which structural changes obstruct the curative properties of a toxin and which are permissible. An athymic mouse model of human malignant mesothelioma (H-Meso) will be used since the natural resistance of murine cells allows diphtheria toxin to selectively kill the human cancer target cells. A single low dose of this toxin rapidly eliminates both the ascites plus solid tumors and consistently cures these mice. Experiments will be performed to determine the pertinent distinctions which explain discrepancies in the action of toxin conjugates and diphtheria toxin in this model. Attention will focus on the pharmacokinetics of the toxins, on their ability to retain full activity in vivo and to reach every cancer cell in the ascites and solid tumors. Various antibodies, antibody fragments and ligands will be linked to diphtheria toxin to determine if such modifications curtail its access to tumor cells and diminish its effectiveness.